While electrophotography can be accomplished by a variety of methods such as those described in U.S. Pat. No. 2,297,691 and Japanese Patent Publication Nos. 23910/67 and 24748/68, the following steps are common to all of the methods currently employed: a latent electric image is formed on a photoreceptor containing a photo-conductive material by various techniques; the latent image is rendered visible with a toner; the toner image is transferred onto a receiving sheet, such as paper, as required; and the transferred image is fixed by suitable means such as heating or application of a solvent vapor so as to obtain a hard copy.
The electric latent image can be made visible with a toner by various techniques such as the magnetic brush method described in U.S. Pat. No. 2,874,063, the cascade development method described in U.S. Pat. No. 2,618,552 and the powder cloud method described in U.S. Pat. No. 2,221,776.
Developers used for this purpose are generally composed of a toner and a carrier in admixture. The toner is prepared by blending in a molten state a resin (e.g., polystyrene, a styrene-butadiene copolymer, and a polyester) with a pigment (e.g., carbon black and phthalocyanine Blue) or a dye being used as colorants, and then grinding the cooled mix into particles ranging from 1 to 30 .mu.m in size. The carrier is in the form of glass beads or particles of metals such as iron, nickel and ferrite having an average size either comparable to that of the toner particles or up to 500 .mu.m, or in the form of such beads or metallic particles provided with coatings of various resins.
Irrespective of the method of development used in practice, actual development is accomplished by the toner which separates from the developer. Since the toner is a component that is directly involved in development, the efficiency with which the electrostatic image on the photoreceptor is visualized and the efficiency with which the developed image is transferred onto a receiving sheet such as paper are of extreme importance for the toner. Other requirements that should be met by the developer are that it produce a uniform charge pattern, that the image quality obtained using the developer is insensitive to environmental factors, and that the developer have high durability. One practice that has often been employed with a view to satisfying these needs is to incorporate a charge controller in the developer as an additive.
However, the conventionally used charge controllers are not completely satisfactory in their ability to meet the aforementioned requirements and they have often failed to attain the desired results.